Presentation on theme: "The Halogens Chapter 40. F Cl Br I At Gp VII Position in Periodic Table."— Presentation transcript:
The Halogens Chapter 40
F Cl Br I At Gp VII Position in Periodic Table
General information HalogenAtomic radius (nm) Ionic radius (nm) Melting Point ( o C) Boiling Point ( o C) Density (g cm -3 ) Abundance on Earth (%) Fluorine Chlorine Bromine Iodine Astatine Trace
Characteristic properties Very reactive non-metals –High electronegativity –High electron affinity Bonding and Oxidation states Colour
Electronegativity A measure of the attracting ability of bonding electrons. Due to their large effective nuclear charge, halogens are the most electronegative element in the periodic table. ElementEN value F4.0 Cl3.0 Br2.8 I2.5
Electron Affinity Generally high electron affinity. All are exothermic. X(g) + e - X - (aq) ElementEA kJ/mol F-348 Cl-364 Br-342 I-285
Bonding and Oxidation State ns 2 np 5 Ionic or covalent bond with oxidation state –1 or +1 (F shows –1 only) Except F, all other halogens can expand their octet by using the low-lying, vacant d- orbitals to form bonding. Their oxidation states range from –1 to +7.
Bonding and Oxidation State ns np nd (+5 state) HClO 3, HBrO 3, I 2 O 5 ns np nd (+7 state) Cl 2 O 7, HIO 4
Colour ElementColour In Standard StateIn WaterIn 1,1,1- trichloroethane FPale yellow ClGreenish yellowPale yellowYellow BrReddish brownYellowOrange IViolet blackBrownViolet
Colour White light Iodine
Check point 40-1 E 1 Iodine E 2 Fluorine
Variation in properties of Halogens F Cl Br I At (Temp. o C) b.p. m.p. m.p./b.p. increase as M r increase because Van der Waals Force increase with M r
Electronegativity F Cl Br I F form ionic compound with –1 oxidation state. I shows highly positive oxidation states with more electronegative element. e.g. KIO 4
Electron affinity F Cl Br I EA decrease from Cl to I due to increasing atomic size which lowers the nuclear attraction to the added electron. F has exceptional lower EA because its 2 nd shell is already crowded with 7 e -, the additional e - will experienced a greater e-e repulsion than other halogens.
Variation in Chemical Properties of elements All are good oxidizing agents. (F 2 >Cl 2 >Br 2 >I 2 ) ½ X 2 (std.state) X - (aq) X(g) X - (g) H atom H H EA H hyd H H atom H EA H hyd =++
Oxidizing power Element F Cl Br I H H atom H EA H hyd Oxidizing strength
Oxidizing properties 1.Oxidation of metals a. F 2 oxidizes all metals including Au and Ag. b.Cl 2 oxidizes common metals such as Na and Cu. 2.Reactions with Fe 2+ (aq) 2Fe 2+ + Cl 2 2Fe Cl - 2Fe 2+ + Br 2 2Fe Br - 2Fe I - 2Fe 2+ + I 2 Standard electrode potential (V) Cl 2 +e - Cl Br 2 +e - Br I 2 +e - I Fe 3+ +e - Fe
Oxidizing properties 3. Reaction with phosphorus 2P + 3X 2 2PX 3 2P + 5X 2 2PX 5 F 2 and Cl 2 form PX 5 as they have high oxidizing Strength. They combine with P to exhibit its maximum oxidation state. Br 2 and I 2 form PX 3 only.
Reaction with water Fluorine oxidizes water to form HF and O 2 2F 2 + 2H 2 O 2HF + O 2 Chlorine undergoes disproportionation(self oxidation and reduction) to form HCl and HOCl. Cl 2 + H 2 O HCl + HOCl
Reaction with water A mixture of Cl 2 (aq), HCl(aq) and HOCl(aq) is called chlorine water. OCl -, chloric(I) ion 1.Strong oxidizing agent with bleaching property OCl - + dye (coloured) Cl - + dye (colourless) 2.Unstable to heat and light 2OCl - O 2 + 2Cl -
Reaction with water Br 2 is only slightly soluble in water. Br 2 (aq) + H 2 O (l) HBr(aq) + HOBr(aq) OBr - (aq) is unstable and with bleaching property: 1.2OBr - O 2 + 2Br - 2. OBr - + dye(coloured) Br - + dye(colourless)
Reaction with water I 2 does not react with water and only very slightly soluble in water. I 2 is more soluble in ethanol or in KI(aq). I 2 (aq) + I - (aq) I 3 - (aq)
Reaction with alkalis All halogens react with aqueous alkalis and disproportionate in it. The products depend on temperature and concentration of the alkalis.
Reaction with alkalis Cold dil.NaOH 2F 2 + 2NaOH 2NaF + OF 2 + H 2 O Cl 2 + 2NaOH NaCl + NaOCl + H 2 O Br 2 + 2NaOH NaBr + NaOBr + H 2 O 3NaOBr 2NaBr + NaBrO 3 (3Br 2 + 6NaOH 5NaBr + NaBrO 3 + 3H 2 O I 2 + 2NaOH NaI + NaOI + H 2 O 3NaOI 2NaI+ NaIO 3 (3I 2 + 6NaOH 5NaI + NaIO 3 + 3H 2 O
Reaction with alkalis More concentrated NaOH 2F 2 + 4NaOH 4NaF + O 2 + 2H 2 O Hot concentrated NaOH Cl 2 + 2NaOH NaCl + NaCl + H 2 O 3NaOCl 2NaCl + NaClO 3 (3Cl 2 + 6NaOH 5NaCl + NaClO 3 + 3H 2 O
Reaction with alkalis The reverse reaction of I 2 and KOH is used to prepare standard iodine solution for iodometric titration. Know amount of iodine is generated by dissolving known quantity of KIO 3 in excess KI and H +. 5KI + KIO 3 + 3H + 3I 2 + 6K + + 3H 2 O
Halogen Displacement reactions Oxidizing strength : F 2 >Cl 2 >Br 2 >I 2 Cl 2 + 2Br - 2Cl - + Br 2 Cl 2 + 2I - 2Cl - + I 2 Br 2 + 2I - 2Br - + I 2 Variation in properties of halides
Halide + c. H 2 SO 4 NaCl + H 2 SO 4 NaHSO 4 + HCl 2NaCl + H 2 SO 4 Na 2 SO 4 + HCl (500 o C) 2NaBr + 2H 2 SO 4 2HBr + Na 2 SO 4 2HI + H 2 SO 4 SO 2 + Br 2 + 2H 2 O 2NaI + 2H 2 SO 4 2HI + Na 2 SO 4 8HI + H 2 SO 4 H 2 S + 4I 2 + 4H 2 O Relative reducing power: HCl < HBr < HI
Halide + c. H 3 PO 4 3NaCl + H 3 PO 4 Na 3 PO 4 + 3HCl 3NaBr + H 3 PO 4 Na 3 PO 4 + 3HBr 3NaI + H 3 PO 4 Na 3 PO 4 + 3HI H 3 PO 4 is NOT a strong oxidizing agent. It reacts with halides to form HX.
Halides + Ag + (aq) Ag + (aq) + Cl - (aq) AgCl(s), white precipitate Ag + (aq) + Br - (aq) AgBr(s), pale yellow precipitate Ag + (aq) + I - (aq) AgI(s), yellow precipitate
Halides + Ag + (aq) IonAction of AgNO 3= Effect of Standing in sunlight Effect of adding excess aq. NH 3 Cl - White ppt.Turns greyWhite ppt. dissolves Br - Pale yellow ppt.Turns yellowish grey Pale yellow ppt. slightly dissolves I-I- Yellow ppt.Remains yellow Yellow ppt. does not dissolve
Relative acidity of HX Hydrogen HalideDissociation constantDegree of dissociation in 0.1M solution HF7x HCl1x HBr1x HI1x
Relative Acidity of HX HX(aq)H + (aq) + X - (aq) HX(g) H + (g) + X - (g) H (g) + X (g) H H 1 H 5 H 6 H 3 H 4 H 2
Relative Acidity of HX HX HF HCl HBr HI H 1 H 2 H 3 H 4 H 5 H 6 HT S G Explanation of weak acid strength of HF: HF has the greatest bond dissociation energy and exceptionally small electron affinity. It has the least exothermic H. 2.Due to formation of strong hydrogen bond and greatest degree of hydration, HF has the smallest decrease of T S.
Anomalous behaviour of HF According to La Chateliers Principle, the degree of dissociation of a weak acid increases with dilution. HA H + + X - However, the acidity of conc. HF is greater than dilute HF. Why? In conc. HF, HF molecules form dimers H 2 F 2 by hydrogen bond. The acid strength of H 2 F 2 is stronger than HF. H 2 F 2 H + + HF 2 -
Uses of Halogens and Halides Teflon Na 2 SiF 6 or NaF is used to fluoridate drinking water. Chlorine bleach, sterilization of water, PVC. Anti-knocking agent (leaded petrol) AgBr coating in film Iodine tincture AgI in coating film